Method and apparatus for measuring turbine shell clearance

ABSTRACT

An apparatus for measuring turbine rotor-to-stator clearances and a method for assembling a turbomachine based on the measured clearances are disclosed. In an embodiment, at least one clearance sensor is inserted into a stator of a turbomachine. Using the sensor, tops-on clearance between a rotor blade tip and an inner surface of a stator is measured while an upper stator shell, a rotor, a lower stator shell are assembled together; and a tops-off clearance is measured while the lower stator shell and a rotor are assembled together. A tops-on/tops-off shift, i.e., a difference between the tops-on clearance and the tops-off clearance, is determined. The turbine can be assembled by adjusting a relative position of the rotor and stator to account for the tops-on/tops-off shift.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of currently pending U.S. patentapplication Ser. No. 13/207,536 filed on Aug. 11, 2011. The applicationidentified above is incorporated herein by reference in its entirety forall that it contains in order to provide continuity of disclosure.

BACKGROUND OF THE INVENTION

The disclosure relates generally to turbo-machines such as steam and gasturbines, and more particularly, to an apparatus and method formeasuring deflection between rotating turbine blade tips and theirsurrounding casing.

Turbomachines, such as gas and steam turbines, typically include acentrally-disposed rotor that rotates within a stator. A working fluidflows through one or more rows of circumferentially arranged rotatingblades that extend radially outward from the rotor shaft. The fluidimparts energy to the shaft, which is used to drive a load such as anelectric generator or compressor.

Clearance between radially outer tips of the rotating blades andstationary shrouds on an interior of the stator in, e.g., compressor andturbine sections of gas turbines strongly impacts efficiency of the gasturbine engine. The smaller the clearance between the rotor blades andthe inner surface of the stator, the lower the likelihood of fluidleakage across blade tips. Fluid leakage across blade tips causes fluidto bypass a row of blades, reducing efficiency.

Insufficient clearance may also be problematic, however. Operatingconditions may cause blades and other components to experience thermalexpansion, which may result in variations in blade tip clearance. Thespecific effects of various operating conditions on blade clearance mayvary depending on the type and design of a particular turbomachine. Forexample, tip clearances in gas turbine compressors may reach their nadirvalues when the turbine is shut down and cooled, whereas tip clearancesin low pressure steam turbines may reach their nadir values duringsteady state full load operation. If insufficient tip clearance isprovided when the turbomachine is assembled or re-assembled afterinspection/repair, the rotating blades may hit the surrounding shroud,causing damage to the shroud on the stator interior, the blades, or bothwhen operating under certain conditions.

During turbine assembly and re-assembly after inspection/repair, thelower stator shell is typically assembled first, then the rotor is setin place. Then the upper stator shell is assembled, including affixingthe upper shell to the lower shell of the stator as shown in FIG. 1.This may typically be done by, e.g., bolting arm 222 of upper statorshell 220 to arm 242 of lower stator shell 240 together in a horizontaljoint 230.

Although rotor-to-stator clearances can be measured in the lower halfprior to assembling the upper half (i.e., in the “tops-off” condition,see FIG. 4), these values are not directly representative of the valuesin the fully assembled turbine (i.e., in “tops-on” condition, see FIG.3) because the turbine shell is supported differently when the uppershell 220 of the stator is affixed to the lower shell 240. In thetops-on condition, support is shifted from the lower shell arm 242 tothe upper shell arms 222, the weight of the upper shell 220 of thestator is added, and when the horizontal joint 230 is bolted, theoverall stator 200 structure stiffens. As a result of these and otherchanges, the rotor-to-stator clearance is different in the tops-on andtops-off conditions, by a factor which may not be readily predictable.In the tops-on condition, in which the turbomachine is operated,clearances cannot be measured directly, since the turbomachine is fullyassembled, and the rotating blades and inner surface 210 of stator 200are not accessible.

One way the tops-on/tops-off shift has been addressed has been to useclearances between the rotating blade tips and the inner surface of thestator that are sufficiently large as to include the tops-on/tops-offdeviation. For reasons discussed above, however, this is detrimental toturbomachine performance and efficiency because it is likely to resultin excessive clearances and leakage of working fluid across bladestages.

Another approach has been to use factory tops-on/tops-off data in thefield. However, this presents a data management problem, as factory datamay be taken years before the turbomachine is disassembled in the fieldand must be reassembled. Differences in conditions between the factoryand the field further complicate this approach.

BRIEF DESCRIPTION OF THE INVENTION

A first aspect of the disclosure provides an apparatus comprising: atleast one sensor inserted in a stator, for measuring a tops-on clearancebetween a rotor blade tip and an inner surface of a stator while anupper stator shell, a rotor, a lower stator shell are assembledtogether, and a tops-off clearance between the rotor blade tip and theinner surface of a stator while the lower stator shell and a rotor areassembled together; and a computing device operably connected with theat least one sensor for determining a tops-on/tops-off shift, whereinthe tops-on/tops-off shift is a difference between the tops-on clearanceand the tops-off clearance.

A second aspect of the disclosure provides a turbomachine comprising: arotor; and a stator surrounding the rotor, the stator including a lowerstator shell and an upper stator shell. At least one sensor is insertedin the lower stator shell, for measuring a tops-on clearance between arotor blade tip and an inner surface of the stator while the upperstator shell, the rotor, and the lower stator shell are assembledtogether, and a tops-off clearance between the rotor blade tip and theinner surface of the stator while the lower stator shell and the rotorare assembled together; and a computing device is operably connectedwith the at least one sensor for determining a tops-on/tops-off shift,wherein the tops-on/tops-off shift is a difference between the tops-onclearance and the tops-off clearance.

A third aspect of the disclosure provides a method for assembling aturbomachine, comprising: using at least one sensor inserted in astator, measuring a tops-on clearance between a rotor blade tip and aninner surface of a stator while an upper stator shell, a rotor, a lowerstator shell are assembled together, and measuring a tops-off clearancebetween the rotor blade tip and the inner surface of a stator while thelower stator shell and a rotor are assembled together; determining atops-on/tops-off shift, wherein the tops-on/tops-off shift is adifference between the tops-on clearance and the tops-off clearance;assembling the lower stator shell; placing the rotor on the lower statorshell at a position shifted from a desired rotor position by a distanceequal to the tops-on/tops-off shift; and assembling the upper statorshell to the lower stator shell.

These and other aspects, advantages and salient features of theinvention will become apparent from the following detailed description,which, when taken in conjunction with the annexed drawings, where likeparts are designated by like reference characters throughout thedrawings, disclose embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded perspective view of an upper and lower shell ofa stator.

FIG. 2 shows a cross sectional schematic view of a turbomachine.

FIG. 3 shows a cross sectional representation of a turbomachine in thetops-on condition in accordance with an embodiment of the invention.

FIG. 4 shows a cross sectional representation of a turbomachine in thetops-off condition in accordance with an embodiment of the invention.

FIG. 5 shows a cross sectional view of a rotor-to-stator clearancedistance in accordance with an embodiment of the invention.

FIG. 6 shows a cross sectional view of a rotor-to-stator clearancedistance in accordance with an embodiment of the invention.

FIG. 7 shows a cross sectional view of a clearance sensor and aclearance sensor retainer member in accordance with an embodiment of theinvention.

FIG. 8 shows a perspective view of a portion of clearance sensorretainer member in accordance with an embodiment of the invention.

FIG. 9 is a flow chart depicting a method in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

At least one embodiment of the present invention is described below inreference to its application in connection with the operation of aturbomachine. Although embodiments of the invention are illustratedrelative to a turbomachine in the form of a steam turbine, it isunderstood that the teachings are equally applicable to otherturbomachines, including but not limited to gas turbines. Further, atleast one embodiment of the present invention is described below inreference to a nominal size and including a set of nominal dimensions.However, it should be apparent to those skilled in the art that thepresent invention is likewise applicable to any suitable turbine and/orgenerator. Further, it should be apparent to those skilled in the artthat the present invention is likewise applicable to various scales ofthe nominal size and/or nominal dimensions.

As indicated above, FIGS. 1-8 depict, and aspects of the inventionprovide, an apparatus for measuring deflection, and FIG. 9 depicts amethod for assembling a turbomachine using the same.

FIGS. 1-2 show different aspects of turbine 100 (labeled in FIG. 2) inaccordance with embodiments of the disclosure. FIG. 1 shows an explodedperspective view of an outer shell of stator 200, which includes upperstator shell 220 and lower stator shell 240. Upper stator shell 220includes an upper stator shell arm 222; lower stator shell 240 likewiseincludes lower stator shell arm 242. As shown in FIG. 2, stator 200surrounds rotor 120, which rotates about a longitudinal axis 250 withinstator 200.

As shown in FIG. 3, lower stator shell 240 includes at least oneclearance sensor 300 inserted therein. Clearance sensor 300 may beinserted in lower stator shell 240 such that clearance sensor 300 isembedded in lower stator shell 240 with a radially outer edge ofclearance sensor 300 substantially flush with an inner surface 210 ofthe stator 200. In some embodiments, as shown in FIGS. 3-4, clearancesensor 300 is located at a bottom dead-center position in the lowershell 240 of stator 200. In other embodiments, clearance sensor 300 maybe offset from a bottom dead center position by a margin of degreeswhich may be accounted for in calculations. Clearance sensor 300 is usedfor measuring clearances 310, 320 (FIGS. 3, 4 respectively) between atip of blade 140 on rotor 120 (FIG. 5), i.e., the radially outermostpoint on rotor 120, and an inner surface 210 of stator 200. As shown inFIG. 5, the radially outermost point on rotor 120 blade 140 may be ablade seal tooth tip 160.

In further embodiments, such as the embodiment shown in FIG. 6,clearance sensor 300 may comprise a plurality of clearance sensors 300.In the embodiment in FIG. 6, clearance sensors 300 are separated by twostages of blades. In other embodiments, between about 3 and about 6clearance sensors 300 may be axially spaced along stator 200. In furtherembodiments, a plurality of clearance sensors 300 may be arranged suchthat one clearance sensor 300 is axially aligned with each of aplurality of stages of blades on rotor 120. In such an embodiment, thenumber of clearance sensors 300 may be equal to the number of stages ofblades on rotor 120. In other arrangements, one clearance sensor 300 maybe axially aligned with every other stage of blades on rotor 120, suchthat the number of clearance sensors 300 may be equal to half of thenumber of stages of blades on rotor 120. These arrangements are merelyillustrative, however; other arrangements of clearance sensors 300relative to stages of blades on rotor 120 are contemplated as otherembodiments of the invention.

As further shown in FIG. 7, clearance sensor 300 may be mounted tostator 200 and held in place by means of sensor retainer member 330.Sensor retainer member 330 may be substantially tube-shaped, with apassageway therein for clearance sensor instrumentation leads 340, and aflange member 331 at a radially outward end relative to turbine 200. Insome embodiments, sensor retainer member 330 may comprise a singlemember; in other embodiments sensor retainer member 330 may comprise twoseparate members, each including a semi-annular portion and portion offlange member 331 such that they can be inserted into stator 200separately and joined together to position clearance sensor 300 andcontain clearance sensor instrumentation leads 340. Bolts 370 may beused to affix flange member 331 of sensor retainer 330 to stator 200.

In order to avoid a potential steam leakage path 380 along sensorretainer member 330, clearance sensor 300 may be either permanentlyaffixed in a manner that fully seals the interface (e.g., welded,brazed, cemented, etc.) or may be installed with enough contact surfacearea and contact force so as to prevent leakage along path 380. In theembodiment shown in FIGS. 7-8, substantially annularly shaped sealingmember 385 includes surface 390, which acts as a sealing surface.Surface 390 is a substantially annularly shaped surface at the distalend, i.e., the end nearer clearance sensor 300.

A proximal end 305 of clearance sensor 300 mates with a surface 390(FIG. 7), and the surfaces are forced together to prevent leakage ofworking fluid in the turbine. Retainer member 330 and the bolts 370 orother method of affixation provide the force necessary to ensure aproper seal. Force may also be applied using other types of springs orfluid systems, e.g., hydraulic or pneumatic. Gaskets or other sealingdevices may also be used to provide a seal.

In embodiments in which turbine 100 is single-shell construction, theclearance sensor 300 may be embedded in the shell or the nozzle ring. Ineither case, the clearance sensor 300 and related hardware (including,e.g., sensor retainer member 330) would penetrate the shell. Inembodiments in which turbine 100 has double-shell construction, theclearance sensor 300 could be embedded in the inner shell (or nozzlecarrier), as shown here in FIG. 7, or in a nozzle outer ring. In such anembodiment, the inner shell would be penetrated by the clearance sensor300 and related hardware, and clearance sensor instrumentation leads 340would exit turbine 100 through an instrumentation port in the outershell.

Referring back to FIG. 3, clearance sensor 300 may measure a tops-onclearance 310, which is the clearance between rotor 120 and innersurface 210 as measured while upper stator shell 220, rotor 120, andlower stator shell 240 are assembled together. Clearance sensor 300 mayalso measure tops-off clearance 320 (FIG. 4), which is the clearancebetween rotor 120 and inner surface 210 as measured while lower statorshell 240 and rotor 120 are assembled together. In some embodiments,clearance sensor 300 may be a voltage drop sensor, and may measure avoltage drop across a clearance 310, 320 between a tip of sensor 300 anda point on rotor 120. Other types of sensors, either now known or laterdeveloped, may also be used.

Clearance sensor 300 may further be in signal communication withcomputing device 350 via clearance sensor instrumentation leads 340.Upon measuring a clearance 310, 320, clearance sensor 300 may transmit asignal representing the clearance 310, 320 to computing device 350. Asshown in FIG. 3, computing device 350 includes a processing unit 346, amemory 352, and input/output (I/O) interfaces 348 operably connected toone another by pathway 354, which provides a communications link betweeneach of the components in computing device 350. Further, computingdevice 350 is shown in communication with display 356, external I/Odevices/resources 358, and storage unit 360. I/O resources/devices 358can comprise one or more human I/O devices, such as a mouse, keyboard,joystick, numeric keypad, or alphanumeric keypad or other selectiondevice, which enable a human user to interact with computing device 350and/or one or more communications devices to enable a device user tocommunicate with computing device 350 using any type of communicationslink. Computing device 350 is shown in phantom in FIG. 4 for purposes ofbrevity only.

In general, processing unit 346 executes computer program code 362 whichprovides the functions of computing device 350. Modules, such as shiftcalculator module 364, which is described further herein, are stored inmemory 352 and/or storage unit 360, and perform the functions and/orsteps of the present invention as described herein. Memory 352 and/orstorage unit 360 can comprise any combination of various types ofcomputer readable data storage media that reside at one or more physicallocations. To this extent, storage unit 360 could include one or morestorage devices, such as a magnetic disk drive or an optical disk drive.Still further, it is understood that one or more additional componentsnot shown in FIG. 3 can be included in computing device 350.Additionally, in some embodiments one or more external devices 358,display 356, and/or storage unit 360 could be contained within computingdevice 350, rather than externally as shown, in the form of a computingdevice 350 which may be portable and/or handheld.

Computing device 350 can comprise one or more general purpose computingarticles of manufacture capable of executing program code, such asprogram 362, installed thereon. As used herein, it is understood that“program code” means any collection of instructions, in any language,code or notation, that cause a computing device having an informationprocessing capability to perform a particular action either directly orafter any combination of the following: (a) conversion to anotherlanguage, code or notation; (b) reproduction in a different materialform; and/or (c) decompression. To this extent, program 362 can beembodied as any combination of system software and/or applicationsoftware.

Further, program 362 can be implemented using a module such as shiftcalculator 364 or set of modules 366. In this case, calculator 364 canenable computing device 350 to perform a set of tasks used by program362, and can be separately developed and/or implemented apart from otherportions of program 362. As used herein, the term “component” means anyconfiguration of hardware, with or without software, which implementsthe functionality described in conjunction therewith using any solution,while the term “module” means program code that enables a computingdevice 350 to implement the actions described in conjunction therewithusing any solution. When fixed in memory 352 or storage unit 360 of acomputing device 350 that includes a processing unit 346, a module is asubstantial portion of a component that implements the actions.Regardless, it is understood that two or more components, modules,and/or systems may share some/all of their respective hardware and/orsoftware. Further, it is understood that some of the functionalitydiscussed herein may not be implemented or additional functionality maybe included as part of computing device 350.

When computing device 350 comprises multiple computing devices, eachcomputing device can have only a portion of program 362 fixed thereon(e.g., one or more modules 364, 366). However, it is understood thatcomputing device 350 and program 362 are only representative of variouspossible equivalent computer systems that may perform a processdescribed herein. To this extent, in other embodiments, thefunctionality provided by computing device 350 and program 362 can be atleast partially implemented by one or more computing devices thatinclude any combination of general and/or specific purpose hardware withor without program code, including but not limited to a handheldmeasuring device for stator-to-rotor clearance. In each embodiment, thehardware and program code, if included, can be created using standardengineering and programming techniques, respectively.

When computing device 350 includes multiple computing devices, thecomputing devices can communicate over any type of communications link.Further, while performing a process described herein, computing device350 can communicate with one or more other computer systems using anytype of communications link. In either case, the communications link cancomprise any combination of various types of wired and/or wirelesslinks; comprise any combination of one or more types of networks; and/orutilize any combination of various types of transmission techniques andprotocols.

As noted, computing device 350 includes a shift calculator module 364for analyzing a signal provided by clearance sensor 300. Using a signalfrom clearance sensor 300 representing a tops-on clearance 310 and asignal representing tops-off clearance 320, shift calculator module 364may calculate a tops-on/tops-off shift. The tops-on/tops-off shift isequal to the difference between tops-on clearance 310 and tops-offclearance 320, and represents the shift in position attributable toinstalling upper stator shell 220 to lower stator shell 240.

Tops-on clearance 310 may be measured when turbomachine 100 is shutdownand cool. In further embodiments, rotor 120 may be rotated on a turninggear during measurement of tops-on clearance 310. This allows clearance310 to account for any variations in clearance related to variations inradially extending length of blades on rotor 120. When measuringtops-off clearance 320, a motor such as, e.g., an air motor, may be usedto rotate rotor 120 for the same purpose. During measurement of tops-offclearance 320, rotor 120 is rotated slowly. For example, rotor 120 maybe rotated at a speed of one half of a rotation per minute.

The measurements of tops-on clearance 310 and tops-off clearance 320 asdescribed above may be used in a method for assembling a turbomachine100. Referring to FIG. 9, in step S1, using clearance sensor (orsensors) 300 inserted in lower stator shell 240 of stator 200, tops-onclearance 310 may be measured while upper stator shell 220, rotor 120, alower stator shell 240 are assembled together (FIG. 3). In step S2,tops-off clearance 320 may be measured while lower stator shell 240 androtor 120 are assembled together (FIG. 4). It is noted that steps S1 andS2 may be performed with either step S1 prior to S2, or the reverse,with step S2 prior to step S1.

In step S3, using computing device 350, including shift calculatormodule 364 as described above, a tops-on/tops-off shift may bedetermined. The tops-on/tops-off shift is equal to the differencebetween tops-on clearance 310 and tops-off clearance 320.

Where, for example, turbomachine 100 had been disassembled formaintenance and/or repair, it may be reassembled by first assemblinglower stator shell 240 (step S4), and placing rotor 120 on lower statorshell 240 (step S5). As discussed above, however, rotor 120 is notplaced such that rotor 120 is in the desired rotor position relative tolower stator shell 240, i.e., tops-off clearance 320 is not equal to theclearance that results in maximal efficiency of turbomachine 100.Rather, rotor 120 is placed in position relative to lower stator shell240 such that it is shifted from the desired rotor position by adistance equal to the tops-on/tops-off shift. Where a plurality ofclearance sensors 300 are used, the relative positions of rotor 120 andlower stator shell 240 are adjusted such that at each axial location ofa clearance sensor 300, rotor 120 is shifted by the tops-on/tops-offshift as described above.

Adjustments in the relative positions of rotor 120 and lower statorshell 240 in order to achieve the appropriate shift from the desiredrotor position may be made in a variety of ways. In one embodiment, theposition of rotor 120 may be adjusted relative to lower stator shell 240in accordance with the tops-on/tops-off shift. Such manipulation ofrotor 120 may be accomplished by, e.g., adjusting the rotor bearings. Inanother embodiment, lower stator shell 240 may be adjusted. Lower statorshell 240 may be manipulated by, e.g., shimming or adjusting statorcomponents including but not limited to nozzles 180 and other statorcomponents. Each nozzle stage 180 (see FIGS. 5-7) may be individuallyadjustable. Where nozzles 180 are not individually adjustable, a bestfit may be used, based on measured data from clearance sensor 300.

In step S6, upper stator shell 220 is assembled to lower stator shell240. As the weight of upper stator shell 220 is added, and horizontaljoint 230 between upper and lower stator shells 220, 240 is affixed by,e.g., bolts at horizontal joint 230, rotor 120 is shifted such that itis positioned relative to inner surface 210 of stator 200 such that whenoperated, it will produce maximal efficiency without impacting innersurface 210 of stator 200.

As previously mentioned and discussed further herein, the apparatus formeasuring deflection, including clearance sensor 300, has the technicaleffect of enabling measurement of the tops-on clearance 310 and tops-offclearance 320 between rotor 120 and stator using clearance sensor 300.Using the measured tops-on 310 and tops-off 320 clearances, atops-on/tops-off shift can be calculated. This tops-on/tops-off shiftmay be used to assemble or re-assemble turbomachine 100 by placing rotor120 on lower stator shell 240, shifted from the desired position(relative to lower stator shell 240) by a distance equal to thetops-on/tops-off shift. When upper stator shell 220 is affixed to lowerstator shell 240, the resulting rotor 120 position will be as desired.It is understood that some of the various components shown in FIG. 3 canbe implemented independently, combined, and/or stored in memory for oneor more separate computing devices that are included in computing device350.

As used herein, the terms “first,” “second,” and the like, do not denoteany order, quantity, or importance, but rather are used to distinguishone element from another, and the terms “a” and “an” herein do notdenote a limitation of quantity, but rather denote the presence of atleast one of the referenced item. The modifier “about” used inconnection with a quantity is inclusive of the stated value and has themeaning dictated by the context (e.g., includes the degree of errorassociated with measurement of the particular quantity). The suffix“(s)” as used herein is intended to include both the singular and theplural of the term that it modifies, thereby including one or more ofthat term (e.g., the metal(s) includes one or more metals). Rangesdisclosed herein are inclusive and independently combinable (e.g.,ranges of “up to about 6, or, more specifically, about 3 to about 6sensors,” is inclusive of the endpoints and all intermediate values ofthe ranges of “about 3 to about 6,” etc.).

While various embodiments are described herein, it will be appreciatedfrom the specification that various combinations of elements, variationsor improvements therein may be made by those skilled in the art, and arewithin the scope of the invention. In addition, many modifications maybe made to adapt a particular situation or material to the teachings ofthe invention without departing from essential scope thereof. Therefore,it is intended that the invention not be limited to the particularembodiment disclosed as the best mode contemplated for carrying out thisinvention, but that the invention will include all embodiments fallingwithin the scope of the appended claims.

What is claimed is:
 1. An apparatus comprising: at least one sensorinserted in a stator, the at least one sensor being configured tomeasure: a tops-on clearance between a rotor blade tip and an innersurface of a stator, wherein in the tops-on condition, the clearance ismeasured while an upper stator shell and a lower stator shell areassembled together, and a rotor is installed within the assembledstator, and a tops-off clearance between the rotor blade tip and theinner surface of the stator while the lower stator shell and the rotorare assembled together, and the upper stator shell is not affixed to thelower stator shell; and a computing device operably connected with theat least one sensor, the computing device being configured to determinea tops-on/tops-off shift, wherein the tops-on/tops-off shift is equal toa difference between the tops-on clearance and the tops-off clearance.2. The apparatus of claim 1, wherein the at least one sensor furthercomprises between about 3 and about 6 sensors axially spaced along thestator.
 3. The apparatus of claim 1, wherein the at least one sensorfurther comprises a plurality of sensors spaced such that one sensor isaxially aligned with each of a plurality of stages of rotor blades. 4.The apparatus of claim 1, wherein the sensor further comprises a voltagedrop sensor for measuring a voltage drop across a clearance between atip of the sensor and a point on the rotor.
 5. The apparatus of claim 1,wherein the sensor is substantially vertically aligned with alongitudinal axis of the rotor.
 6. A turbomachine comprising: a rotor; astator surrounding the rotor, the stator including a lower stator shelland an upper stator shell, a plurality of sensors inserted in the lowerstator shell, each of the plurality of sensors being configured tomeasure: a tops-on clearance between a rotor blade tip and an innersurface of the stator, wherein in the tops-on condition, the clearanceis measured while the upper stator shell and the lower stator shell areassembled together and the rotor is installed within the assembledstator, and a tops-off clearance between the rotor blade tip and theinner surface of the stator while the lower stator shell and the rotorare assembled together, and the upper stator shell is not affixed to thelower stator shell; and a computing device operably connected to each ofthe plurality of sensors, the computing device being configured todetermine a tops-on/tops-off shift, wherein the tops-on/tops-off shiftis equal to a difference between the tops-on clearance and the tops-offclearance; wherein the plurality of sensors are spaced such that either:one sensor is axially aligned with each of a plurality of stages ofrotor blades, or one sensor is axially aligned with every other stage ofthe plurality of stages of rotor blades.
 7. The turbomachine of claim 6,wherein the plurality of sensors further comprises between about 3 andabout 6 sensors axially spaced along the stator.
 8. The turbomachine ofclaim 6, wherein the plurality of sensors further comprises a pluralityof voltage drop sensors for measuring a voltage drop across a clearancebetween a tip of each sensor and a point on the rotor.
 9. Theturbomachine of claim 6, wherein the plurality of sensors aresubstantially vertically aligned with a longitudinal axis of the rotor.10. The turbomachine of claim 6, wherein the tops-on clearance furthercomprises the tops-on clearance when the turbomachine is shutdown andcool.
 11. The turbomachine of claim 6, further comprising a turning gearfor rotating the rotor during measuring of the tops-on clearance. 12.The turbomachine of claim 6, further comprising a motor for rotating therotor during measuring of the tops-off clearance.
 13. The turbomachineof claim 12, wherein the rotating is at a speed of about 0.5 rotationper minute.
 14. The turbomachine of claim 6, wherein the plurality ofsensors inserted in the lower stator shell further are embedded in aninner stator shell such that a radially outer edge of each of theplurality of sensors is substantially flush with the inner surface ofthe stator.
 15. The turbomachine of claim 6, wherein each of theplurality of sensors are affixed to the lower stator shell by a sensorretainer member.